Wireless communication technology has experienced unprecedented growth, fueled by advances in radio frequency, satellite, and microelectronic technologies and by the convenience of access to telephony and portable wireless devices. Several technologies have emerged for providing wireless communications, including analog systems such as Advanced Mobile Phone System (AMPS), and digital systems such as Global System for Mobile Communications (GSM) and Digital AMPS (D-AMPS). Cellular radio systems operate in an interference-limited environment and typically rely on frequency reuse plans and Frequency Division Multiple Access (FDMA), in analog systems, and Time Division Multiple Access (TDMA), in digital systems, for example, to maximize capacity.
Because bandwidth and radio spectrum are limited resources shared by all users in a wireless network, schemes to divide the available network bandwidth among as many subscribers as possible are often employed. One such scheme involves a combination Time and Frequency Division Multiple Access (TDMA/FDMA) method. For example, 25 MHZ bandwidth can be divided into 124 carrier frequencies spaced 200 kHz apart, with one or more carrier frequencies assigned to each Base Station in the network. Each of the carrier frequencies, in turn, can be divided in time using a TDMA scheme to define logical channels. Channels, in turn, may be divided into dedicated subchannels which are allocated to a mobile station with common channels used by mobile stations in the idle mode.
A Traffic Channel (TCH) is used to carry speech and data signals. Traffic channels are defined in GSM using a 26 multi-frame, or a group of 26 TDMA frames. With 26 frames, 24 can be used for traffic, one for a control channel, and one unused. In other systems, other methods are used, such as CDMA or FDMA. An Idle Channel Measurement (ICM) algorithm that uses dynamic channel assignment is usually utilized by the Base Station Controller (BSC) to select channels for transmission of speech and data signals.
In addition to dividing bandwidth among subscribers, transmission channels are assigned or allocated to incoming calls to the network. Two methods of channel assignment are typically used in the industry: fixed channel assignment and dynamic channel assignment. Fixed channel assignment involves assigning a predetermined set of channels or frequencies to the base station or BSC. If all the channels within the cell of the base station or BSC are occupied, either the call is blocked, meaning the mobile handset does not receive service, or borrowing strategies are used to borrow channels from neighboring cell. With dynamic channel assignment, channels are not allocated to a base station or BSC permanently. The Mobile Switching Center (MSC) assigns the BSC some frequencies, which remain fixed for a period of time. Periodically, some measurements are made regarding channel utilization, and adjustments are made to the channel assignments as needed. The frequencies or channels are assigned by the BSC each time a call request is made.
A problem with prior art channel assignment algorithms and devices is that channel assignments are made based on the best interference level available. This results in an unbalanced use of traffic channels, with all voice and data calls being assigned to the best quality transmission channels even when a lower quality channel is available to service a call. Continuous monitoring of transmission channel interference levels and improved assignment algorithms would provide a more balanced network which could be utilized by a greater number of subscribers.